Search for new phenomena in final states with large jet multiplicities and missing transverse momentum at root s=8 TeV proton-proton collisions using the ATLAS experiment

JHEP10(2013)130

Published for SISSA by Springer

Received: August 8, 2013 Revised: September 16, 2013 Accepted: September 27, 2013 Published: October 21, 2013

Search for new phenomena in final states with large

jet multiplicities and missing transverse momentum at

√

s = 8 TeV proton-proton collisions using the

ATLAS experiment

The ATLAS collaboration

E-mail:

atlas.publications@cern.ch

Abstract: A search is presented for new particles decaying to large numbers (7 or more) of

jets, with missing transverse momentum and no isolated electrons or muons. This analysis

uses 20.3 fb

−1

of pp collision data at

√

s = 8 TeV collected by the ATLAS experiment at

the Large Hadron Collider. The sensitivity of the search is enhanced by considering the

number of b-tagged jets and the scalar sum of masses of large-radius jets in an event. No

evidence is found for physics beyond the Standard Model. The results are interpreted in

the context of various simplified supersymmetry-inspired models where gluinos are pair

produced, as well as an mSUGRA/CMSSM model.

JHEP10(2013)130

Contents

1

Introduction

1

2

The ATLAS detector and data samples

3

3

Physics object selection

4

4

Event selection

5

4.1

The multi-jet + flavour stream

6

4.2

The multi-jet + M

_JΣ

stream

6

4.3

Summary of signal regions

6

5

Standard model background determination

8

5.1

Monte Carlo simulations

8

5.2

Multi-jet background

9

5.3

Systematic uncertainties in the multi-jet background determination

10

5.4

Leptonic backgrounds

12

5.5

Systematic uncertainties in the leptonic background determination

16

6

Results

17

6.1

Simultaneous fit in the multi-jet + flavour stream

19

6.2

Simultaneous fit in the multi-jet + M

_JΣ

stream

22

6.3

Fit results

22

7

Interpretation

22

8

Conclusion

29

The ATLAS collaboration

34

1

Introduction

Many extensions of the Standard Model of particle physics predict the presence of

TeV-scale strongly interacting particles that decay to weakly interacting descendants. In the

context of R-parity-conserving supersymmetry (SUSY) [

1

–

5

], the strongly interacting

par-ent particles are the partners of the quarks (squarks, ˜

q) and gluons (gluinos, ˜

g), and are

produced in pairs. The lightest supersymmetric particle (LSP) is stable, providing a

candi-date that can contribute to the relic dark-matter density in the universe [

6

,

7

]. If they are

kinematically accessible, the squarks and gluinos could be produced in the proton-proton

interactions at the Large Hadron Collider (LHC) [

8

].

JHEP10(2013)130

Such particles are expected to decay in cascades, the nature of which depends on the

mass hierarchy within the model. The events would be characterised by significant missing

transverse momentum from the unobserved weakly interacting descendants, and by a large

number of jets from emissions of quarks and/or gluons. Individual cascade decays may

include gluino decays to a top squark (stop, ˜

t) and an anti-top quark,

˜

g → ˜

t + ¯

t

(1.1a)

followed by the top-squark decay to a top quark and a neutralino LSP, ˜

χ

0₁

,

˜

t → t + ˜

χ

01

.

(1.1b)

Alternatively, if the top squark is heavier than the gluino, the three-body decay,

˜

g → t + ¯

t + ˜

χ

01

(1.2)

may result. Other possibilities include decays involving intermediate charginos, neutralinos,

and/or squarks including bottom squarks.

A pair of cascade decays produces a large

number of Standard Model particles, together with a pair of LSPs, one from the end of

each cascade. The LSPs are assumed to be stable and only weakly interacting, and so

escape undetected, resulting in missing transverse momentum.

In this paper we consider final states with large numbers of jets together with significant

missing transverse momentum in the absence of isolated electrons or muons, using the pp

collision data recorded by the ATLAS experiment [

9

] during 2012 at a centre-of-mass energy

of

√

s = 8 TeV. The corresponding integrated luminosity is 20.3 fb

−1

. Searches for new

phenomena in final states with large jet multiplicities — requiring from at least six to at

least nine jets — and missing transverse momentum have previously been reported by the

ATLAS Collaboration using LHC pp collision data corresponding to 1.34 fb

−1

[

10

] and to

4.7 fb

−1

[

11

] at

√

s = 7 TeV. Searches with explicit tagging of jets from bottom quarks

(b-jets) in multi-jet events were also performed by ATLAS [

12

] and CMS [

13

–

15

]. These

searches found no significant excess over the Standard Model expectation and provide

limits on various supersymmetric models, including decays such as that in eq. (

1.2

) and an

mSUGRA/CMSSM [

16

–

21

] model that includes strong production processes. The analysis

presented in this paper extends previous analyses by reaching higher jet multiplicities and

utilizing new sensitive variables.

Events are first selected with large jet multiplicities, with requirements ranging from at

least seven to at least ten jets, reconstructed using the anti-k

t

clustering algorithm [

22

,

23

]

and jet radius parameter R = 0.4. Significant missing transverse momentum is also required

in the event. The sensitivity of the search is further enhanced by the subdivision of the

selected sample into several categories using additional information. Event clasification

based on the number of b-jets gives enhanced sensitivity to models which predict either

more or fewer b-jets than the Standard Model background. In a complementary stream

of the analysis, the R = 0.4 jets are clustered into large (R = 1.0) composite jets to form

an event variable, the sum of the masses of the composite jets, which gives additional

JHEP10(2013)130

containing isolated, high transverse-momentum (p

T

) electrons or muons are vetoed in order

to reduce backgrounds involving leptonic W boson decays. The previous analyses [

10

,

11

] had signal regions with smaller jet multiplicities; those are now omitted since the

absence of significant excesses in earlier analyses places stringent limits on models with

large cross sections.

Searches involving final states with many jets and missing transverse momentum have

been confirmed to have good sensitivity to decays such as those in eqs. (

1.1

) and (

1.2

) [

11

],

but they also provide sensitivity to any model resulting in final states with large jet

mul-tiplicity in association with missing transverse momentum. Such models include the pair

production of gluinos, each of them decaying via an off-shell squark, as

˜

g → ¯

q + q

0

+ ˜

χ

±1

→ ¯

q + q

0

+ W

±

+ ˜

χ

01

,

(1.3a)

or alternatively

˜

g → ¯

q + q

0

+ ˜

χ

±1

→ ¯

q + q

0

+ W

±

+ ˜

χ

0 2

→ ¯

q + q

0

+ W

±

+ Z

0

+ ˜

χ

01

.

(1.3b)

Another possibility is the pair production of gluinos which decay as in eq. (

1.1a

) and the

subsequent decay of the ˜

t-squark via

˜

t → b + ˜

χ

±₁

,

or via the R-parity-violating decay

˜

t → ¯

b + ¯

s.

(1.4)

Several supersymmetric models are used to interpret the analysis results: simplified

models that include decays such as those in eqs. (

1.1

)–(

1.4

), and an mSUGRA/CMSSM

model with parameters

1

tan β = 30, A

0

= −2m

0

and µ > 0, which accommodates a lightest

Higgs boson mass compatible with the observed Higgs boson mass at the LHC [

25

,

26

].

2

The ATLAS detector and data samples

The ATLAS experiment is a multi-purpose particle physics detector with a

forward-backward symmetric cylindrical geometry and nearly 4π coverage in solid angle.

2

The

layout of the detector is defined by four superconducting magnet systems, which

com-prise a thin solenoid surrounding the inner tracking detectors (ID), and a barrel and two

end-cap toroids generating the magnetic field for a large muon spectrometer. The ID

provides precision reconstruction of tracks in the region |η| < 2.5. The calorimeters lie

1

A particular mSUGRA/CMSSM model point is specified by five parameters: the universal scalar mass m0, the universal gaugino mass m1/2, the universal trilinear scalar coupling A0, the ratio of the vacuum expectation values of the two Higgs fields tan β, and the sign of the higgsino mass parameter µ.

2

ATLAS uses a right-handed coordinate system with its origin at the nominal interaction point (IP) in the centre of the detector and the z-axis along the beam pipe. The x-axis points from the IP to the centre of the LHC ring, and the y-axis points upward. Cylindrical coordinates (r, φ) are used in the transverse plane, φ being the azimuthal angle around the beam pipe. The pseudorapidity is defined in terms of the polar angle θ as η = − ln tan(θ/2), and the transverse energy ET by ET= E sin θ.

JHEP10(2013)130

between the ID and the muon system.

In the pseudorapidity region |η| < 3.2,

high-granularity liquid-argon (LAr) electromagnetic (EM) sampling calorimeters are used. An

iron/scintillator-tile calorimeter provides hadronic coverage for |η| < 1.7. The end-cap and

forward regions, spanning 1.5 < |η| < 4.9, are instrumented with LAr calorimeters for both

EM and hadronic measurements.

The data sample used in this analysis was taken during the period from March to

December 2012 with the LHC operating at a pp centre-of-mass energy of

√

s = 8 TeV.

Ap-plication of data-quality requirements results in an integrated luminosity of 20.3 ± 0.6 fb

−1

,

where the luminosity is measured using techniques similar to those described in ref. [

27

],

with a preliminary calibration of the luminosity scale derived from beam-overlap scans

performed in November 2012. The analysis makes use of dedicated multi-jet triggers, the

final step of which required either at least five jets with E

T

> 55 GeV or at least six jets

with E

T

> 45 GeV, where the jets must have |η| < 3.2. The final level of the trigger

selection is based on a jet algorithm and calibration method closely matched to those used

in the signal region selections. In all cases the trigger efficiency is greater than 99% for

events satisfying the jet multiplicity selection criteria for the signal regions described in

section

4

. Events selected with single-lepton triggers and prescaled multi-jet triggers are

used for background determination in control regions.

3

Physics object selection

Jets are reconstructed using the anti-k

t

jet clustering algorithm with radius parameter R =

0.4. The inputs to this algorithm are the energies and positions of clusters of calorimeter

cells, where the clusters are formed starting from cells with energies significantly above the

noise level [

28

]. Jet momenta are constructed by performing a four-vector sum over these

clusters of calorimeter cells, treating each as an (E, p) four-vector with zero mass. The

local cluster weighting (LCW) calibration method [

29

] is used to classify clusters as being of

either electromagnetic or hadronic origin and, based on this classification, applies specific

energy corrections derived from a combination of Monte Carlo simulation and data [

28

].

A further calibration is applied to the corrected jet energies to relate the response of the

calorimeter to the true jet energy [

28

]. The jets are corrected for energy from additional

proton-proton collisions (pile-up) using a method, proposed in ref. [

30

], which estimates the

pile-up activity in any given event, as well as the sensitivity of any given jet to pile-up. The

method subtracts a contribution from the jet energy equal to the product of the jet area

and the average energy density of the event. All jets are required to satisfy p

T

> 20 GeV

and |η| < 2.8. More stringent requirements on p

T

and on |η| are made when defining signal

regions as described in section

4

.

Jets with heavy-flavour content are identified using a tagging algorithm that uses

both impact parameter and secondary vertex information [

31

]. This b-tagging algorithm

is applied to all jets that satisfy both |η| < 2.5 and p

T

> 40 GeV. The parameters of the

algorithm are chosen such that 70% of b-jets and about 1% of light-flavour or gluon jets

are selected in t¯

t events in Monte Carlo simulations [

32

]. Jets initiated by charm quarks

are tagged with about 20% efficiency.

JHEP10(2013)130

Electrons are required to have p

T

> 10 GeV and |η| < 2.47.

They must satisfy

‘medium’ electron shower shape and track selection criteria based upon those described

in ref. [

33

], but modified to reduce the impact of pile-up and to match tightened

trig-ger requirements. They must be separated by at least ∆R = 0.4 from any jet, where

∆R =

p(∆η)

2

_{+ (∆φ)}

2

_{. Events containing electrons passing these criteria are vetoed}

when forming signal regions. Additional requirements are applied to electrons when

defin-ing leptonic control regions used to aid in the estimate of the SM background contributions,

as described in section

5.4

; in this case, electrons must have p

T

> 25 GeV, must satisfy the

‘tight’ criteria of ref. [

33

], must have transverse and longitudinal impact parameters within

5 standard deviations and 0.4 mm, respectively, of the primary vertex, and are required to

be well isolated.

3

Muons are required to have p

T

> 10 GeV and |η| < 2.5, to satisfy track quality selection

criteria, and to be separated by at least ∆R = 0.4 from the nearest jet candidate. Events

containing muons passing these criteria are vetoed when forming signal regions. When

defining leptonic control regions, muons must have p

T

> 25 GeV, |η| < 2.4, transverse and

longitudinal impact parameters within 5 standard deviations and 0.4 mm, respectively, of

the primary vertex and they must be isolated.

4

The missing transverse momentum two-vector p

miss

T

is calculated from the negative

vector sum of the transverse momenta of all calorimeter energy clusters with |η| < 4.5

and of all muons [

34

]. Clusters associated with either electrons or photons with p

T

>

10 GeV, and those associated with jets with p

T

> 20 GeV and |η| < 4.5 make use of

the calibrations of these respective objects. For jets the calibration includes the area-based

pile-up correction described above. Clusters not associated with such objects are calibrated

using both calorimeter and tracker information. The magnitude of p

_Tmiss

, conventionally

denoted by E

_Tmiss

, is used to distinguish signal and background regions.

4

Event selection

Following the physics object reconstruction described in section

3

, events are discarded

if they contain any jet that fails quality criteria designed to suppress detector noise and

non-collision backgrounds, or if they lack a reconstructed primary vertex with five or more

associated tracks. Events containing isolated electron or muon candidates are also vetoed

as described in section

3

. The remaining events are then analysed in two complementary

analysis streams, both of which require large jet multiplicities and significant E

_Tmiss

. The

selections of the two streams are verified to have good sensitivity to decays such as those

in eqs. (

1.1

)–(

1.4

), but are kept generic to ensure sensitivity in a broad set of models with

large jet multiplicity and E

_Tmiss

in the final state.

3_{The electron isolation requirements are based on nearby tracks and calorimeter clusters, as follows.} The scalar sum of transverse momenta of tracks, other than the track from the electron itself, in a cone of radius ∆R = 0.3 around the electron is required to be smaller than 16% of the electron’s pT. The scalar sum of calorimeter transverse energy around the electron in the same cone, excluding the electron itself, is required to be smaller than 18% of the electron’s pT.

4_{The scalar sum of the transverse momenta of the tracks, other than the track from the muon itself,} within a cone of ∆R = 0.3 around the muon must be less than 12% of the muon’s pT, and the scalar sum of calorimeter transverse energy in the same cone, excluding that from the muon, must be less than 12% of the muon’s p .

JHEP10(2013)130

4.1

The multi-jet + flavour stream

In the multi-jet + flavour stream the number of jets with |η| < 2 and p

T

above the

threshold p

min_T

= 50 GeV is determined. Events with exactly eight or exactly nine such

jets are selected, and the sample is further subdivided according to the number of the jets

(0, 1 or ≥2) with p

T

> 40 GeV and |η| < 2.5 which satisfy the b-tagging criteria. The

b-tagged jets may belong to the set of jets with p

T

greater than p

minT

, but this is not a

requirement. Events with ten or more jets are retained in a separate category, without any

further subdivision.

A similar procedure is followed for the higher jet-p

T

threshold of p

minT

= 80 GeV. Signal

regions are defined for events with exactly seven jets or at least eight jets. Both categories

are again subdivided according to the number of jets (0, 1 or ≥2) that are b-tagged. Here

again, the b-tagged jets do not necessarily satisfy the p

min_T

requirement.

In all cases the final selection variable is E

_Tmiss

/

√

H

T

, the ratio of the E

miss_T

to the

square root of the scalar sum H

T

of the transverse momenta of all jets with p

T

> 40 GeV

and |η| < 2.8. This ratio is closely related to the significance of the E

miss

T

relative to

the resolution due to stochastic variations in the measured jet energies [

34

]. The value of

E

_Tmiss

/

√

H

T

is required to be larger than 4 GeV

1/2

for all signal regions.

4.2

The multi-jet + M

_JΣ

stream

Analysis of the multi-jet + M

_JΣ

stream proceeds as follows. The number of (R = 0.4) jets

with p

T

above 50 GeV is determined, this time using a larger pseudorapidity acceptance

of |η| < 2.8. Events with at least eight, at least nine or at least ten such jets are retained,

and a category is created for each of those multiplicity thresholds. The four-momenta of

the R = 0.4 jets satisfying p

T

> 20 GeV and |η| < 2.8 are then used as inputs to a second

iteration of the anti-k

t

jet algorithm, this time using the larger distance parameter R = 1.0.

The resulting larger objects are denoted as composite jets. The selection variable M

_JΣ

is

then defined to be the sum of the masses m

R=1.0_j

of the composite jets

M

_JΣ

≡

X

j

m

R=1.0_j

,

where the sum is over the composite jets that satisfy p

R=1.0_T

> 100 GeV and |η

R=1.0

| < 1.5.

Signal regions are defined for two different M

_JΣ

thresholds. Again the final selection requires

that E

miss_T

/

√

H

T

> 4 GeV

1/2

.

4.3

Summary of signal regions

The nineteen resulting signal regions are summarized in table

1

. Within the multi-jet +

flavour stream the seven signal regions defined with p

min_T

= 50 GeV are mutually disjoint.

The same is true for the six signal regions defined with the threshold of 80 GeV. However,

the two sets of signal regions overlap; an event found in one of the p

min_T

= 80 GeV signal

regions may also be found in one of the p

min_T

= 50 GeV signal regions. The multi-jet +

M

_JΣ

stream has six inclusive signal regions; for example an event which has at least ten

R = 0.4 jets with p

T

> 50 GeV, M

JΣ

> 420 GeV and E

Tmiss

/

√

H

T

> 4 GeV

1/2

will be found

in all six multi-jet + M

_JΣ

regions. These overlaps are treated in the results of the analysis

as described in section

6

.

JHEP10(2013)130

Multi-jet

+

fla

v

our

stream

Multi-jet

+

M

Σ J

stream

Iden

tifier

8j50

9j50

≥

10j50

7j80

≥

8j80

≥

8j50

≥

9j50

≥

10j50

Jet

|η

|

<

2

.0

<

2

.0

<

2

.8

Jet

p

T

>

50

Ge

V

>

80

Ge

V

>

50

Ge

V

Jet

coun

t

=

8

=

9

≥

10

=

7

≥

8

≥

8

≥

9

≥

10

b-jets

0

1

≥

2

0

1

≥

2

—

0

1

≥

2

0

1

≥

2

—

(p

T

>

40

Ge

V

,

|η

|

<

2

.5)

M

Σ J

[GeV]

—

—

>

340

and

>

420

for

eac

h

case

E

miss T

/

√

H

T

>

4

Ge

V

1 / 2

>

4

Ge

V

1 / 2

>

4

Ge

V

1 / 2 T able 1 . Definition of the nineteen signal regions. The jet |η |, pT and m ultiplicit y all refer to the R = 0 .4 jets. Comp osite jets with the larger radius parameter R = 1 .0 are used in the m ulti-jet + M Σ J stream when constructing M Σ J . A long dash ‘—’ indicates that n o requiremen t is made.

JHEP10(2013)130

5

Standard model background determination

Two background categories are considered in this search: (1) multi-jet production,

in-cluding purely strong interaction processes and fully hadronic decays of t¯

t, and hadronic

decays of W and Z bosons in association with jets, and (2) processes with leptons in the

final states, collectively referred to as leptonic backgrounds. The latter consist of

semilep-tonic and fully lepsemilep-tonic decays of t¯

t, including t¯

t production in association with a boson;

leptonically decaying W or Z bosons produced in association with jets; and single top

quark production.

The major backgrounds (multi-jet, t¯

t, W + jets, and Z + jets) are determined with the

aid of control regions, which are defined such that they are enriched in the background

process(es) of interest, but nevertheless remain kinematically close to the signal regions.

The multi-jet background determination is fully data-driven, and the most significant of the

other backgrounds use data control regions to normalise simulations. The normalisations of

the event yields predicted by the simulations are adjusted simultaneously in all the control

regions using a binned fit described in section

6

, and the simulation is used to extrapolate

the results into the signal regions. The methods used in the determination of the multi-jet

and leptonic backgrounds are described in sections

5.2

and

5.4

, respectively.

5.1

Monte Carlo simulations

Monte Carlo simulations are used as part of the leptonic background determination

pro-cess, and to assess the sensitivity to specific SUSY signal models. Most of the leptonic

backgrounds are generated using Sherpa-1.4.1 [

35

] with the CT10 [

36

] set of parton

dis-tribution functions (PDF). For t¯

t production, up to four additional partons are modelled

in the matrix element. Samples of W + jets and Z + jets events are generated with up to

five additional partons in the matrix element, except for processes involving b-quarks for

which up to four additional partons are included. In all cases, additional jets are generated

via parton showering. The leptonic W + jets, Z + jets and t¯

t backgrounds are normalised

according to their inclusive theoretical cross sections [

37

,

38

]. In the case of t¯

t

produc-tion, to account for higher-order terms which are not present in the Sherpa Monte Carlo

simulation, the fraction of events initiated by gluon fusion, relative to other processes, is

modified to improve the agreement with data in t¯

t-enriched validation regions described

in section

5.4

. This corresponds to applying a scale factor of 1.37 to the processes

initi-ated by gluon fusion and a corresponding factor to the other processes to keep the total t¯

t

cross section the same. The estimation of the leptonic backgrounds in the signal regions is

described in detail in section

5.4

.

Smaller background contributions are also modelled for the following processes:

sin-gle top quark production in association with a W boson in the s-channel (MC@NLO

4.06 [

39

–

42

] / Herwig 6.520 [

43

] / Jimmy 4.31 [

44

]), t-channel single top quark

produc-tion (AcerMC3.8 [

45

] / Pythia-6.426 [

46

]), and t¯

t production in association with a W

or Z boson (Madgraph-5.1.4.8 [

47

] / Pythia-6.426).

Supersymmetric production processes are generated using Herwig++2.5.2 [

48

] and

JHEP10(2013)130

to next-to-leading order in the strong coupling constant α

S

, including the resummation of

soft gluon emission at next-to-leading-logarithmic accuracy (NLO+NLL) [

50

–

54

].

For each process, the nominal cross section and its uncertainty are taken from an

envelope of cross-section predictions using different PDF sets and factorisation and

renor-malisation scales, as described in ref. [

55

]. All Monte Carlo simulated samples also include

simulation of pile-up and employ a detector simulation [

56

] based on GEANT4 [

57

]. The

simulated events are reconstructed with the same algorithms as the data.

5.2

Multi-jet background

The dominant background at intermediate values of E

_Tmiss

is multi-jet production including

purely strong interaction processes and fully hadronic decays of t¯

t. The contribution from

these processes is determined using collision data and the selection criteria were designed

such that multi-jet processes can be accurately determined from supporting measurements.

The background determination method is based on the observation that the E

_Tmiss

resolution of the detector is approximately proportional to

√

H

T

and almost independent

of the jet multiplicity in events dominated by jet activity, including hadronic decays of

top quarks and gauge bosons [

10

,

11

]. The distribution of the ratio E

_Tmiss

/

√

H

T

therefore

has a shape that is almost invariant under changes in the jet multiplicity. The multi-jet

backgrounds can be determined using control regions with lower E

_Tmiss

/

√

H

T

and/or lower

jet multiplicity than the signal regions. The control regions are assumed to be dominated

by Standard Model processes, and that assumption is corroborated by the agreement with

Standard Model predictions of multi-jet cross-section measurements for up to six jets [

58

].

Events containing heavy quarks show a different E

_Tmiss

/

√

H

T

distribution than those

containing only light-quark or gluon jets, since semileptonic decays of heavy quarks contain

neutrinos. The dependence of E

_Tmiss

/

√

H

T

on the number of heavy quarks is accounted for

in the multi-jet + flavour signal regions by using a consistent set of control regions with the

same b-jet multiplicity as the target signal distribution. The E

_Tmiss

/

√

H

T

distribution is also

found to be approximately independent of the M

_JΣ

event variable, so a similar technique

is used to obtain the expected multi-jet background contributions to the multi-jet + M

_JΣ

signal regions.

The leading source of variation in E

_Tmiss

/

√

H

T

under changes in the jet multiplicity

comes from a contribution to E

miss

T

from calorimeter energy deposits not associated with

jets and hence not contributing to H

T

. The effect of this ‘soft’ energy is corrected for by

reweighting the E

_Tmiss

/

√

H

T

distribution separately for each jet multiplicity in the signal

region, to provide the same

P E

CellOut

T

/H

T

distribution, where

P E

CellOutT

is the scalar sum

of E

T

over all clusters of calorimeter cells not associated with jets having p

T

> 20 GeV or

electron, or muon candidates.

For example, to obtain the multi-jet contribution to the multi-jet + flavour stream

9j50 signal region with exactly one b-jet, the procedure is as follows. A template of the

shape of the E

_Tmiss

/

√

H

T

distribution is formed from events which have exactly six jets with

p

T

> 50 GeV, and exactly one b-jet (which is not required to be one of the six previous

jets). The expected contribution from leptonic backgrounds is then subtracted, so that

the template provides the expected distribution resulting from the detector resolution,

JHEP10(2013)130

together with any contribution to the resolution from semileptonic b-quark decays. The

nine-jet background prediction for the signal region (E

_Tmiss

/

√

H

T

> 4 GeV

1/2

) with exactly

one b-jet is then given by

N

_predictedmulti-jet

=



N

_dataA, njet=9

− N

A, n_{leptonic MC}jet=9



×





N

_dataB, njet=6

− N

B, n_{leptonic MC}jet=6

N

_dataA, njet=6

− N

A, n_{leptonic MC}jet=6





,

(5.1)

where A ≡ E

miss T

/

√

H

T

< 1.5 GeV

1/2

, B ≡ E

Tmiss

/

√

H

T

> 4 GeV

1/2

, and each of the

counts N is determined after requiring the same b-jet multiplicity as for the target signal

region (i.e. exactly one b-jet in this example). Equation

5.1

is applied separately to each

of ten bins (of width 0.1) in

P E

CellOut

T

/H

T

to find the prediction for that bin, and then

the contributions of the ten bins summed to provide the

P E

CellOut

T

/H

T

-weighted multi-jet

prediction.

An analogous procedure is used to obtain the expected multi-jet contribution to each

of the other multi-jet + flavour stream signal regions by using the appropriate p

min_T

, jet

multiplicity, and b-jet multiplicity as required by the target signal region. In each case

the shape of the E

_Tmiss

/

√

H

T

distribution is obtained from a ‘template’ with exactly six

(five) jets for signal regions with p

min_T

= 50 (80) GeV. The distributions of E

_Tmiss

/

√

H

T

for

multi-jet + flavour stream control regions are shown in figure

1

.

The procedure in the multi-jet + M

_JΣ

stream is similar: the same jet p

min_T

, jet

multi-plicity and M

_JΣ

criteria are used when forming the template and control regions that are

required for the target signal region. E

miss

T

/

√

H

T

distributions for control regions with

exactly seven jets with p

T

>50 GeV and additional M

_JΣ

selection criteria applied are shown

in figure

2

. Leptonic backgrounds are subtracted, and

P E

CellOut

T

/H

T

weighting is applied.

For all cases in the multi-jet + M

_JΣ

stream the E

_Tmiss

/

√

H

T

template shape is determined

from a sample which has exactly six jets with p

T

> 50 GeV.

Variations in the shape of the E

miss_T

/

√

H

T

distribution under changes in the jet

mul-tiplicity are later used to quantify the systematic uncertainty associated with the method,

as described in section

5.3

.

5.3

Systematic uncertainties in the multi-jet background determination

The multi-jet background determination method is validated by measuring the accuracy of

the predicted E

_Tmiss

/

√

H

T

template for regions with jet multiplicities and/or E

_Tmiss

/

√

H

T

smaller than those chosen for the signal regions. The consistency of the prediction with

the number of observed events (closure) is tested in regions with E

miss

T

/

√

H

T

[ GeV

1/2

] in

the ranges (1.5, 2.0), (2.0, 2.5), and (2.5, 3.5) for jet multiplicities of exactly seven, eight

and nine, and in the range (1.5, 2.0) and (2.0, 3.5) for ≥10 jets. The tests are performed

separately for 0, 1 and ≥ 2 b-tagged jets. In addition, the method is tested for events with

exactly six (five) jets with p

min_T

= 50 GeV (80 GeV) across the full range of E

_Tmiss

/

√

H

T

in

this case using a template obtained from events with exactly five (four) jets. The five-jet

(four-jet) events are obtained using a prescaled trigger for which only a fraction of the total

luminosity is available. Agreement is found both for signal region jet multiplicities at

inter-mediate values of E

_Tmiss

/

√

H

T

and also for the signal region E

Tmiss

/

√

JHEP10(2013)130

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 -1 L dt = 20.3 fb ∫ = 8 TeV s No b-jets > 50 GeV T 7 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (a) No b-jets 0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 1 b-jet > 50 GeV T 7 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2

(b) Exactly one b-jet

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 2 b-jets ≥ > 50 GeV T 7 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2

(c) At least two b-jets

Figure 1. Distribution of E_Tmiss/√HT for the control regions with exactly seven jets with pT≥

50 GeV and |η| < 2.0, for different b-jet multiplicities. The multi-jet prediction is determined from an Emiss

T /

√

HT template obtained from events with exactly six jets. It is normalised to the data

in the region Emiss T /

√

HT< 1.5 GeV1/2 after subtraction of the leptonic backgrounds. The most

important leptonic backgrounds are also shown, based on Monte Carlo simulations. Variable bin sizes are used with bin widths (in units of GeV1/2) of 0.5 (up to Emiss

T /

√

HT= 4 GeV1/2), 1 (from

4 to 6), 2 (from 6 to 8) and 4 thereafter. For reference and comparison, a supersymmetric model is used where gluinos of mass 900 GeV are pair produced and each decay as in eq. (1.2) to a t¯t pair and a ˜χ01 with a mass of 150 GeV. The model is referred to as ‘[˜g, ˜χ01] : [900, 150] [GeV]’.

JHEP10(2013)130

1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 7 jets, p 340 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52 (a) MJΣ≥ 340 GeV 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 7 jets, p 420 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52 (b) MJΣ≥ 420 GeV

Figure 2. Distribution of E_Tmiss/√HT for control regions with exactly seven jets with pT ≥ 50

GeV, and satisfying the same requirements as the multi-jet + MΣ

J stream signal regions, other than

that on Emiss T /

√

HTitself. The multi-jet prediction was determined from an ETmiss/

√

HT template

obtained from events with exactly six jets. Other details are as for figure1.

multiplicity. A symmetrical systematic uncertainty on each signal region is constructed by

taking the largest deviation in any of the closure regions with the same jet multiplicity or

lower, for the same b-tagging requirements. Typical closure uncertainties are in the range

5% to 15%; they can grow as large as ∼50% for the tightest signal regions, due to larger

statistical variations in the corresponding control regions.

Additional systematic uncertainties result from modelling of the heavy-flavour

con-tent (25%), which is assessed by using combinations of the templates of different b-tagged

jet multiplicity to vary the purity of the different samples. The closure in simulation of

samples with high heavy-flavour content is also tested. The leptonic backgrounds that are

subtracted when forming the template have an uncertainty associated with them (5–20%,

depending on the signal region). Furthermore, other uncertainties taken into account are

due to the scale choice of the cutoff for the soft energy term,

P E

CellOut

T

, (3–15%) and the

trigger efficiency (<1%) in the region where the template is formed.

5.4

Leptonic backgrounds

The leptonic backgrounds are defined to be those which involve the leptonic decays W →

`ν or Z → νν. Contributions are determined for partly hadronic (i.e. semileptonic or

dileptonic) t¯

t, single top, W and Z production, and diboson production, each in association

with jets. The category excludes semileptonic decays of charm and bottom quarks, which

are considered within the multi-jet category (section

5.2

). The leptonic backgrounds which

contribute most to the signal regions are t¯

t and W + jets. In each case, events can evade

the lepton veto, either via hadronic τ decays or when electrons or muons are produced but

not reconstructed.

JHEP10(2013)130

Single-lepton validation region

Lepton pT > 25 GeV

Lepton multiplicity Exactly one, ` ∈ {e, µ} Emiss T > 30 GeV Emiss T / √ HT > 2.0 GeV1/2 mT < 120 GeV Jet pT

Jet multiplicity As for signal regions (table1) b-jet multiplicity

MΣ J

Control region (additional criteria) Jet multiplicity Unit increment if p`

T> p min T Emiss T / q HT (+p`T) > 4.0 GeV 1/2

Table 2. The selection criteria for the validation and control regions for the t¯t and W + jets backgrounds. In the control region the lepton is recast as a jet so it contributes to HTif p`T> 40 GeV

and to the jet multiplicity count if p`

T> pminT .

The predictions employ the Monte Carlo simulations described in section

5.1

. When

predictions are taken directly from the Monte Carlo simulations, the leptonic background

event yields are subject to large theoretical uncertainties associated with the use of a

leading-order Monte Carlo simulation generator. These include scale variations as well

as changes in the number of partons present in the matrix element calculation, and

un-certainties in the response of the detector. To reduce these unun-certainties the background

predictions are, where possible, normalised to data using control regions and cross-checked

against data in other validation regions. These control regions and validation regions are

designed to be distinct from, but kinematically close to, the signal regions, and orthogonal

to them by requiring an identified lepton candidate.

The validation and control regions for the t¯

t and W + jets backgrounds are defined in

table

2

. In single-lepton regions, a single lepton (e or µ) is required, with sufficient p

T

to

allow the leptonic trigger to be employed. Modest requirements on E

_Tmiss

and E

_Tmiss

/

√

H

T

reduce the background from fake leptons. An upper limit on

m

T

=

q

2 |p

miss

T

||p

`T

| − p

Tmiss

· p

`T

,

where p

`_T

is the transverse momentum vector of the lepton, decreases possible

contamina-tion from non-Standard-Model processes.

Since it is dominantly through hadronic τ decays that W bosons and t¯

t pairs contribute

to the signal regions, the corresponding control regions are created by recasting the muon

or electron as a jet. If the electron or muon has sufficient p

T

(without any additional

cali-bration), it is considered as an additional ‘jet’ and it can contribute to the jet multiplicity

count, as well as to H

T

and hence to the selection variable E

Tmiss

/

√

JHEP10(2013)130

2 3 4 5 6 7 8 9 10 11 12 13 14 Events -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 11 10 Data Total background ql,ll → t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z Single top +W,Z t t ]:[900,150] [GeV] 0 1 χ ∼ , g ~ [ -1 L dt = 20.3 fb ∫ = 8 TeV s 1 lepton CR No b-jets ATLAS >50 GeV T Number of jets p 2 3 4 5 6 7 8 9 10 11 12 13 Data/Prediction 0 0.5 1 1.5 2 (a) No b-jets 2 3 4 5 6 7 8 9 10 11 12 13 14 Events -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 10 10 Data Total background ql,ll → t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z Single top +W,Z t t ]:[900,150] [GeV] 0 1 χ ∼ , g ~ [ -1 L dt = 20.3 fb ∫ = 8 TeV s 1 lepton CR 1 b-jet ATLAS >50 GeV T Number of jets p 2 3 4 5 6 7 8 9 10 11 12 13 Data/Prediction 0 0.5 1 1.5 2

(b) Exactly one b-jet

2 3 4 5 6 7 8 9 10 11 12 13 14 Events -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 9 10 Data Total background ql,ll → t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z Single top +W,Z t t ]:[900,150] [GeV] 0 1 χ ∼ , g ~ [ -1 L dt = 20.3 fb ∫ = 8 TeV s 1 lepton CR 2 b-jets ≥ ATLAS >50 GeV T Number of jets p 2 3 4 5 6 7 8 9 10 11 12 13 Data/Prediction 0 0.5 1 1.5 2 (c) ≥2 b-jets

Figure 3. Jet multiplicity distributions for pmin_T = 50 GeV jets in the one-lepton t¯t and W + jets control regions (CR) for different b-jet multiplicities. Monte Carlo simulation predictions are before fitting to data. Other details are as for figure1. The band in the ratio plot indicates the experimental uncertainties on the Monte Carlo simulation prediction and also includes the Monte Carlo simulation statistical uncertainty. Additional theoretical uncertainties are not shown.

JHEP10(2013)130

Events -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 Data Total background ql,ll → t t + light jets ν l → W + b-jets ν l → W , ll + jets ν ν → Z Single top +W, Z t t ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 1 lepton CR 340 GeV ≥ Σ J M No b-jet >50 GeV T Number of jets p 2 3 4 5 6 7 8 9 10 11 12 13 Data/Prediction 0 0.51 1.52

(a) MJΣ>340 GeV, no b-jets

Events -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 Data Total background ql,ll → t t + light jets ν l → W + b-jets ν l → W , ll + jets ν ν → Z Single top +W, Z t t ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 1 lepton CR 340 GeV ≥ Σ J M 1 b-jet ≥ >50 GeV T Number of jets p 2 3 4 5 6 7 8 9 10 11 12 13 Data/Prediction 0 0.51 1.52 (b) MJΣ>340 GeV, ≥1 b-jets Events / 80 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 Data Total background ql,ll → t t + light jets ν l → W + b-jets ν l → W , ll + jets ν ν → Z Single top +W, Z t t ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 1 lepton CR 7 jets 50 GeV ≥ b-blind [GeV] Σ J

Total ’composite’ jet mass, M 0 100 200 300 400 500 600 700 800 900 1000

Data/Prediction

0 0.51 1.52

(c) MJΣdistribution, ≥7j50 selection applied

Figure 4. Jet multiplicity distributions for pmin_T = 50 GeV jets in the one-lepton t¯t and W + jets control regions (CR) for different b-jet multiplicities and a selection on MΣ

J > 340 GeV (4(a))–

(4(b)), and the MΣ

J distribution for an inclusive selection of seven jets with pminT = 50 GeV (4(c)).

JHEP10(2013)130

Two-lepton validation region

Lepton pT > 25 GeV

Lepton multiplicity Exactly two, e e or µ µ

m`` 80 GeV to 100 GeV

Jet pT

Jet multiplicity As for signal regions (table1) b-jet multiplicity

MΣ J

Control region (additional criteria) |pmiss T + p `1 T+ p `2 T|/ √ HT > 4.0 GeV1/2

Table 3. The selection criteria for the validation and control regions for the Z + jets background.

multiplicity as the signal region is required for the equivalent control regions. Additionally,

the same criteria for E

_Tmiss

/

√

H

T

, M

JΣ

and the number of b-tagged jets are required. For

the M

_JΣ

stream these control regions are further split into regions with no b-tagged jets and

those with b-tagged jets to allow separation of contributions from W +jets and t¯

t events.

Provided the expected number of Standard Model events in the corresponding control

re-gion is greater than two, the number of observed events in that control rere-gion is used in a

fit to determine the Standard Model background as described in section

6

. Distributions

of jet multiplicity for the leptonic control regions can be found in figures

3

–

4

. In figure

4

the M

_JΣ

distribution for a leptonic control region is also shown.

The Z+jets control regions require two same-flavour leptons with an invariant mass

consistent with that of the Z boson. To create control regions that emulate the signal

regions, the lepton transverse momenta are added to the missing momentum two-vector

and then the requirement E

miss_T

/

√

H

T

> 4 GeV

1/2

is applied. This emulates the situation

expected for the Z → νν background. The details of the selection criteria are given in

ta-ble

3

. This selection, but with relaxed jet multiplicity criteria, is used to validate the Monte

Carlo simulation description of this process; however, insufficient events remain at high jet

multiplicity, so the estimation of this background is taken from Monte Carlo simulations.

5.5

Systematic uncertainties in the leptonic background determination

Systematic uncertainties on the leptonic backgrounds originate from both detector-related

and theoretical sources from the Monte Carlo simulation modelling. Experimental

uncer-tainties are dominated by those on the jet energy scale, jet energy resolution and, in the

case of the flavour stream, b-tagging efficiency. Other less important uncertainties result

from the modelling of the pile-up, the lepton identification and the soft energy term in the

E

_Tmiss

calculation; these make negligible contributions to the total systematic uncertainty.

The

ATLAS

jet

energy

scale

and

resolution

are

determined

using

in-situ

techniques [

28

,

59

]. The jet energy scale uncertainty includes uncertainties associated with

the quark-gluon composition of the sample, the heavy-flavour fraction and pile-up

uncer-JHEP10(2013)130

tainties. The uncertainties are derived for R = 0.4 jets and propagated to all objects and

selections used in the analysis. The sources of the jet energy scale uncertainty are treated

as correlated between the various Standard Model backgrounds as well as with the signal

contributions when setting exclusion limits. The uncertainties on the yields due to those

on the jet energy scale and resolution range typically between 20% and 30%. The b-tagging

efficiency uncertainties are treated in a similar way when setting limits and have typical

values of ≈ 10%. They are derived from data samples tagged with muons associated with

jets, using techniques described in refs. [

31

,

32

].

For the t¯

t background, theoretical uncertainties are evaluated by comparing the

particle-level predictions of the nominal Sherpa samples with additional samples in which

some of the parameter settings were varied. These include variations of the factorisation

scale, the matching scale of the matrix element to the parton shower, the number of partons

in the matrix element and the PDFs. Alpgen [

60

] samples are also generated with the

renormalisation scale associated with α

S

in the matrix element calculation varied up and

down by a factor of two relative to the original scale k

t

between two partons [

61

]. Finally,

samples with and without weighting of events initiated by gluon fusion relative to other

processes are used to provide a systematic uncertainty on this procedure. The two latter

sources of systematic uncertainty are the dominant ones with typical values of 25–30%

each, leading to a total theoretical uncertainty on the t¯

t background of ≈ 40%.

Alternative samples are generated similarly for the other smaller backgrounds with

different parameters and/or generators to assess the associated theoretical uncertainties,

which are found to be similar to those for the t¯

t background.

6

Results

Figures

5

–

8

show the E

_Tmiss

/

√

H

T

distributions for all the signal regions of both analysis

streams. In order to check the consistency of the data with the background-only and signal

hypotheses, a simultaneous profile maximum likelihood fit [

62

] is performed in the control

and signal regions, for each of the analysis streams separately. Poisson likelihood functions

are used for event counts in signal and control regions. Systematic uncertainties are treated

as nuisance parameters. They are assumed to follow Gaussian distributions and their effect

is propagated to the likelihood function. A control region is taken into account in the fit

if there are at least two expected events associated with it. The fits differ significantly

between the two analysis streams, as described in the following sections.

When evaluating a supersymmetric signal model for exclusion, any signal

contamina-tion in the control regions is taken into account for each signal point in the control-region

fits performed for each signal hypothesis. Separately, each signal region (one at a time),

along with all control regions, is also fitted under the background-only hypothesis. This

fit is used to characterise the agreement in each signal region with the background-only

hypothesis, and to extract visible cross-section limits and upper limits on the production

of events from new physics. For these limits, possible signal contamination in the control

regions is neglected.

JHEP10(2013)130

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 -1 L dt = 20.3 fb ∫ = 8 TeV s No b-jets > 50 GeV T 8 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (a) 8j50, no b-jets 0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 -1 L dt = 20.3 fb ∫ = 8 TeV s No b-jets > 50 GeV T 9 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (b) 9j50, no b-jets 0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 1 b-jet > 50 GeV T 8 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2

(c) 8j50, exactly one b-jet

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 1 b-jet > 50 GeV T 9 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2

(d) 9j50, exactly one b-jet

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 8 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 2 b-jets ≥ > 50 GeV T 8 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (e) 8j50, ≥ 2 b-jets 0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 2 b-jets ≥ > 50 GeV T 9 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (f) 9j50, ≥ 2 b-jets Figure 5. Emiss T / √

HT distributions for the multi-jet + flavour stream with pminT = 50 GeV, and

either exactly eight jets (left) or exactly nine jets (right) with the signal region selection, other than that on E_Tmiss/√HTitself. The b-jet multiplicity increases from no b-jets (top) to exactly one b-jet

JHEP10(2013)130

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 -1 L dt = 20.3 fb ∫ = 8 TeV s > 50 GeV T 10 jets p ≥ ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 Figure 6. Emiss T / √

HT distribution for the multi-jet + flavour stream with pminT = 50 GeV, and

at least ten jets. The complete ≥ 10j50 selection has been applied, other than the final Emiss T /

√ HT

requirement. Other details are as for figure1.

6.1

Simultaneous fit in the multi-jet + flavour stream

The seven p

min_T

= 50 GeV signal regions (and similarly the six p

min_T

= 80 GeV signal

regions) are fitted to the background and signal predictions. Correlations from sample to

sample and region to region are taken into account, separately for the p

min_T

= 50 GeV and

p

min_T

= 80 GeV signal regions. Systematic uncertainties arising from the same source are

treated as fully correlated.

The fit considers several independent background components:

• t¯

t and W + jets. One control region is defined for each signal region, as described in

table

2

; the normalisation of each background component is allowed to vary freely in

the fit.

• Less significant backgrounds (Z + jets, t¯

t + W , t¯

t + Z, and single top) are determined

using Monte Carlo simulations. These are individually allowed to vary within their

uncertainties.

• Multi-jet background. Being data-driven, it is not constrained in the fit by any

control region. It is constrained in the signal regions by its uncertainties, which are

described in section

5.3

.

The systematic effects, described in sections

5.3

and

5.5

, are treated as nuisance

param-eters in the fit. For the signal, the dominant systematic effects are included in the fit; these

are the jet energy scale and resolution uncertainties, the b-tagging efficiency uncertainties,

and the theoretical uncertainties.

JHEP10(2013)130

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 -1 L dt = 20.3 fb ∫ = 8 TeV s No b-jets > 80 GeV T 7 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (a) 7j80, no b-jets 0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 -1 L dt = 20.3 fb ∫ = 8 TeV s No b-jets > 80 GeV T 8 jets p ≥ ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (b) 8j80, no b-jets 0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 1 b-jet > 80 GeV T 7 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2

(c) 7j80, exactly one b-jet

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 1 b-jet > 80 GeV T 8 jets p ≥ ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2

(d) 8j80, exactly one b-jet

0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 7 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 2 b-jets ≥ > 80 GeV T 7 jets p ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (e) 7j80, ≥ 2 b-jets 0 2 4 6 8 10 12 14 16 1/2 Events / 4 GeV -1 10 1 10 2 10 3 10 4 10 5 10 6 10 -1 L dt = 20.3 fb ∫ = 8 TeV s 2 b-jets ≥ > 80 GeV T 8 jets p ≥ ATLAS Data Total background Multi-jets ql,ll → t t Single top +W,Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 0 1 χ∼ , g ~ [ ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data / Prediction 0 0.5 1 1.5 2 (f) 8j80, ≥ 2 b-jets Figure 7. Emiss T / √

HTdistributions for the multi-jet + flavour stream with pminT = 80 GeV. The

complete signal region selections were applied, other than the final Emiss T /

√

HTrequirement. Other

JHEP10(2013)130

1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 8 jets, p ≥ 340 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 8 jets, p ≥ 420 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 9 jets, p ≥ 340 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 9 jets, p ≥ 420 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 10 jets, p ≥ 340 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52 1/2 Events / 4 GeV -2 10 -1 10 1 10 2 10 3 10 4 10 5 10 6

10 DataTotal background

Multi-jets ql,ll → t t Single top +W, Z t t + b-jets ν l → W + light jets ν l → W , ll + jets ν ν → Z ]:[900,150] [GeV] 1 0 χ ∼ , g ~ [ ATLAS =8 TeV s , -1 L dt = 20.3 fb ∫ 50 GeV ≥ T 10 jets, p ≥ 420 GeV ≥ Σ J M ] 1/2 [GeV T H / miss T E 0 2 4 6 8 10 12 14 16 Data/Prediction 0 0.51 1.52

Figure 8. E_Tmiss/√HT distributions for the multi-jet + MJΣ stream with the signal region

selection, other than the final Emiss T /

√

HT requirement. The figures on the left are for events

with MΣ

J > 340 GeV, while those on the right are for MJΣ> 420 GeV. The minimum multiplicity

requirement for pmin

T = 50 GeV, R = 0.4 jets increases from eight (top) to nine (middle) and finally